(1) Field of the Invention
The present invention relates to a process for preparing a photosensitive heat-resistant polymer. More particularly, the present invention relates to a process for preparing a photosensitive polyimide precursor having excellent shelf stability and high sensitivity and containing less impurities.
(2) Description of the Prior Art
As heat-resistant photosensitive materials, photosensitive polyimides are widely used as raw materials for insulating films of semiconductors and for passivation films. For example, in Japanese Patent Laid-open Publication No. 145794/1979, a method is suggested in which a compound containing a double bond and an amino group or its quaternary salt is mixed with polyamic acid. Furthermore, in Japanese Patent Laid-open Publication Nos. 45746/1980 and 100143/1985, other methods are suggested in which an unsaturated epoxy compound or an isocyanate compound having a double bond is reacted with the carboxyl group of each polyamic acid. Japanese Patent Publication No. 41422/1980 discloses a polymer in which an active functional group such as a double bond is introduced into the ester side chain of polyamic acid. In addition, Japanese Patent Laid-open Publication No. 6729/1985 discloses a method for synthesizing a polyimide by the use of a diamine having a double bond which has been previously synthesized.
The method described in Japanese Patent Laid-open Publication No. 145794/1979 mentioned above has the drawback that since a great deal of the compound containing the amino group or its quaternary salt is added to the unstable polyamic acid solution, the viscosity of the solution changes noticeably with time. The techniques in Japanese Patent Laid-open Publication Nos. 45746/1980 and 100143/1985 have the drawback that when the compound having the photosensitive unsaturated group is reacted with the carboxyl group of the polyamic acid, the viscosity of the solution changes owing to the partial decomposition of the polyamic acid and the like. In Japanese Patent Publication No. 41422/1980, an acid chloride is reacted with a diamine, but after the reaction, the chloride remains as an impurity, and the removal of this impurity is troublesome. Furthermore, in Japanese Patent Laid-open Publication No. 6729/ 1985, the process for the introduction of the photosensitive functional group is complicated, and much cost is required. As understood from the foregoing, the conventional techniques have a variety of the problems, and so it is demanded to develop photosensitive materials without such problems.